What effect does increased calcium ion concentration have on muscle contraction?

Increased calcium ion concentration plays a crucial role in the process of muscle contraction by enhancing the exposure of myosin-binding sites on the actin filaments. When a muscle cell is stimulated, calcium ions are released from the sarcoplasmic reticulum into the cytosol. The rise in calcium concentration leads to the binding of calcium to the troponin complex, which is associated with the actin filaments.

This binding causes a conformational change in the troponin, which then shifts another protein called tropomyosin away from the myosin-binding sites on the actin molecules. With the myosin-binding sites exposed, the myosin heads can attach to actin, initiating the cross-bridge cycle and leading to muscle contraction. Thus, the presence of calcium is essential for enabling actin and myosin to interact, which is fundamental for muscle contraction.